Everybody's a lawyer ...
Oh .... waitaminute ....
Sigh ... I kinda figured I was reaching.
I suppose I could substitute the Zhentarim for Iuz/Xendros and the Red Wizards for the Scarlet Brotherhood ... but ... dang it ... it just works with Iuz and the SB ...
Thank you very much. Now ... off to do the rewrite.
There are a few questions one should ask when wondering whether something opens one up to legal liability:
1) Do I want to be the test case?
2) How much am I willing to pay to find out if I'm right?
3) Does a win in court translate to a win out of court?
As to (1), students are told in law school that every case in a textbook is because someone screwed up in an extremely avoidable way. You don't want to be a case in a textbook.
As to (2), the money you earn from publishing your adventure will be less than your legal fees incurred to defend. The US and UK are extremely different on this. In the UK, the loser pays the winner's legal fees. In the US, unless there is an express agreement to hold harmless or litigation conduct meriting an award of fees as a sanction, parties pay their own legal fees regardless of outcome.
As to (3), for the record, I do debt collection law. It's a less-than-glamorous area of practice that exists where the rubber meets the road.
The DMs Guild license is not the OGL. It's a different license. That said, the DMs Guild license is, at the end of the day, a license. That means that they're letting you use some of their IP but that IP ultimately still belongs to them. The original stuff that you bring to the table - that remains yours. However, you are giving them a license to use your IP that you publish under the DMs Guild license.
And DMs Guild appears to give a slightly enlarged license to use their proprietary monsters (such as mind flayers and beholders).
What does this all mean in a practical sense?
A license is kind of like a ticket. In fact, a ticket is a license. A license to use. If you go to the theatre, your ticket is a license to use a particular seat at a particular show. And, if you don't comply with the theatre's rules, they may be able to revoke that license (kick you out).
So, let's say that under the best case scenario you won in court and the characters you want to use are technically permitted to use those characters. What do you win? A spot on a blacklist.
It sucks. It really does. But it's better to save yourself a lot of grief later by accepting a little grief now.